A vehicular navigation system detects a current position of the vehicle (e.g., using Global Positioning System), displays a map with the current position marked on a display screen, and navigates along an optimum route to a destination. In particular, an intersection on the route is emphasized on the screen while an advancing direction in which the vehicle should advance is indicated.
An intersection may be a rotary. As the vehicle approaches the rotary, a passing direction or advancing direction in which the vehicle should pass through the rotary is indicated using an arrow on the screen and/or via voice (see Patent Document 1).
In Patent Document 1, whether the vehicle travels straight (straight-travel) is determined based on (i) an angle difference between an entering road and exiting road and (ii) a given angle defined as an attachment angle with respect to the entering road and exiting road. For instance, when an angle difference is 30 degrees or less, straight-travel is determined as an advancing direction and guided to a user. When an angle difference is from 30 to 45 degrees, and additionally when an attachment angle is 30 degrees or less, straight-travel is also determined as an advancing direction.
Furthermore, that the vehicle should take a U-turn at the rotary via a one-way road is determined when the following conditions are satisfied at the same time: an angle difference between an entering and exiting roads is from 150 to 210 degrees; the entering and exiting roads are one-way traffic roads with the same name; and connection roads to the entering and exit roads are adjacent two-way traffic roads and have the same road number. In other words, the vehicle travels a connection road, a first one-way traffic road entering the rotary, almost one circuit of a rotary link, a second one-way traffic road exiting from the rotary, and then the connection road again.
Furthermore, angle variations from the first one-way traffic road though the rotary link to the second one-way traffic road are integrated positively (counterclockwise) and negatively (clockwise). The integrated angle θ is categorized into the following and corresponding guidance is provided: −180 degrees<θ<−150 degrees is for returning to right; −150 degrees<θ<0 degree is for turning right; 0 degree<θ<+150 degrees is for turning left; and +150 degrees<θ<+180 degree is for returning to left.
As explained above, the navigation system in Patent Document 1 needs the very complicated computation using various factors or determinations for navigating at the rotary such as straight-travel, U-turn along a one-way traffic road, or right or left turn. Furthermore, guidance provided or indicated at the rotary is sometimes different from what the user actually senses or feels while passing through the rotary.
An example will be explained below with reference to FIG. 13. Roads LA, LB, LC, LD are two-way traffic roads and entering into or exiting from a rotary via bifurcated one-way traffic roads LAi, LAo, LBi, LBo, LCi, LCo, LDi, LDo. A guidance route designated from LA through LAi, a rotary link, and LDo to LD is shown as a bold line in FIG. 13. Here, the above navigation system in Patent Document 1 provides guidance based on an angle α between LAi and LDo to output “sharply right bearing.” However, since LAi and LDo have short distances, the user senses that the vehicle takes a typical right bearing from LA to LD, which makes the user feel discrepancy in the guidance. If a guidance route is from LA through LAi, a rotary link, LCo to LC, the guidance is unnaturally outputted as “slightly right bearing.”                Patent Document 1: JP-H11-51684 A (U.S. Pat. No. 6,108,604)        